This project seeks to develop a minimally invasive electrode array for long term recording of brain activity, with single cell resolution. Multielectrode arrays are an essential tool in experimental neuroscience, yet current arrays are severely limited by a mismatch between large or stiff electrodes and the fragile environment of the brain. Chronically implanted electrodes cause ongoing damage to the brain, and an active process of rejection eventually silences neural signals. Failure of chronic implants over long time-scales makes it very challenging to study the neural basis of learning, and prohibits the implementation of long term stable brain machine interfaces for human patients. To minimize electrode damage, the size of implants must be reduced, but multichannel arrays built from the smallest electrodes are impossible to implant due to buckling of the individual fibers. The proposed electrode array solves this mechanical problem - achieving large channel count and sub-cellular (5 micron) individual electrode size in an bundle that strengthens each fiber through mutual support. During implant, however, the bundle splays apart and each fiber follows its own separate course into the brain, preserving the minimally invasive properties of the single fibers. Chronic recordings from prototype designs reveal stable signals, including multiunit recordings with time-scales of months that show minimal drift in neural firing patterns. This project seeks t document how the electrodes interact with vasculature during implant, what damage they cause over three month time-scales, and how these factors relate to the yield and stability of chronic recordings gathered continuously for three months. The methods involve in-vivo imaging of electrode insertion, chronic recording of neural signals in freely behaving animals, and histological analysis of neuronal health and signs of local immune activation near the implant. The anticipated result is that during insertion, individual fibers travel along their own paths of least resistance into the brain, leading to reduced vascular damage. On the timescales of chronic recordings, the anticipated result is improved tissue health and stable neural signals in close proximity to the electrode. Specific variations in experiments proposed here will inform future designs that seek to scale up the number of channels in the tunneling fiber array, providing an opportunity to track large ensembles of cells simultaneously. The near term application of this project will be seen in small animal studies where it is virtually impossible t track the firing patterns of ensembles of neurons through learning with existing large-scale electrodes. Advances focussed on this deliverable are likely to also translate into more stable recordings in larger organisms, with potential direct benefits to human brain machine interfaces.